MRI is an imaging method which excites nuclear spin of an object set in a static magnetic field with a RF (radio frequency) signal having the Larmor frequency magnetically and reconstructs an image based on MR (magnetic resonance) signals generated due to the excitation.
In MRI, various pre-pulses are applied as RF pulses prior to data acquisition in order to obtain an image having a desired contrast. For example, in MRA (magnetic resonance angiography) method to perform imaging of a blood vessel, a spin labeling pulse (also referred to a tagging pulse) is applied in order to obtain a contrast difference between a blood flow and a background tissue.
The spin labeling pulse is a pre-pulse for tagging spins included in fluid such as blood or CSF (cerebrospinal fluid) which flows into an imaging section. Especially, a spin labeling pulse for spin labeling of blood is called an ASL (Arterial spin labeling) pulse.
A t-SLIP (TIME-SLIP: time-Spatial Labeling Inversion Pulse) is known as a representative spin labeling pulse used in non-contrast-enhanced MRA. The t-SLIP includes a region non-selective IR (inversion recovery) pulse and a region selective IR pulse. It is possible to set a region selective IR pulse arbitrarily independent of an imaging region. Therefore, when NMR signals are suppressed by a region non-selective IR pulse while a labeling region is labeled by a region selective IR pulse so that NMR signals from blood in a specific region is enhanced, blood flowing out of the labeling region after a TI (inversion time) can be selectively depicted as a high signal part.
Note that, a region non-selective IR pulse can be switched between ON/OFF. When a region non-selective IR pulse is set to OFF, signals from blood included in a labeling region are suppressed by application of a region selective IR pulse. Therefore, blood flowing out of the labeling region after a TI is depicted as a low signal region.
In addition to the t-SLIP as mentioned above, a RF pulse such as a saturation pulse is used for adjusting a contrast. A presaturation (Presat) pulse applied as a RF pre-pulse is a pre-pulse for suppressing signals from a desired matter by saturating spins of the desired matter.
RF pre-pulses, including a t-SLIP and a Presat pulse, each applied for adjusting a contrast can be combined mutually. That is, MR images having various contrasts can be acquired by applying plural similar or dissimilar region selective RF pre-pulses or region non-selective RF pre-pulses prior to data acquisition.
When application of a single RF pre-pulse is set at the time of setting imaging conditions, an intended contrast can be grasped easily. That is, at the time of setting imaging conditions, an operator can easily grasp that an image having what contrast is acquired.
However, when application of plural RF pre-pulses including a region selective RF pre-pulse is set, there is a possibility that the intended contrast cannot be grasped easily. For example, when application regions of plural region selective RF pre-pulses are different and there is an overlap section between the application regions, spins included in the overlap section receive influences of the respective region selective RF pre-pulses. In addition, when a 180-degree region non-selective IR pulse is applied, the longitudinal magnetization of spins is inverted by 180 degrees and sections showing high signals and low signals are inverted.
Consequently, there is a possibility that an operator cannot easily grasp contrasts possible in respective sections of an imaging region.
An object of the present invention is to provide a magnetic resonance imaging apparatus and a magnetic resonance imaging method which make it possible to easily grasp an estimated contrast at the time of setting imaging conditions in case of imaging with applying a region selective pulse as a RF pulse for adjusting a contrast.